Speaking to talkSPORT, the ex-Ireland international said Chelsea made a mistake by failing to replace Didier Drogba this summer.
Jose Mourinho brought in Samuel Eto'o from Anzhi, but Cascarino believes the Chelsea boss is unsure who is best striker is.
"I spoke over the summer about the urgency of buying a replacement for Didier Drogba and I thought the best player out there was Benteke," Cascarino told talkSPORT.
"I felt that would have been a massive statement. He's the type of player they needed in the way they play.
"The way Mourinho's teams have played over the years, he loves that physical man, and he doesn't have it. He has three forwards and doesn't know which one to pick.
"It would have made a huge difference [getting Benteke in]."
Eto'o was preferred to Demba Ba and Fernando Torres for last night's Champions League match against Basel.
But the Cameroonian failed to find the net for the second successive match as the Swiss side recorded a famous win at Stamford Bridge.
Benteke, who was one of the surprise packages in the Premier League last term scoring 19 goals, has already hit four in four games this season.